This application claims the priority of German application 100 13 428.9, filed Mar. 17, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for the production of double-walled hollow sections by means of internal high-pressure forming. The invention more specifically relates to a method involving the use of a removable intermediate layer between an inner hollow section and an outer hollow section during a single internal high-pressure forming stage.
A method of the generic type is known from DE 197 52 772 A1. In this method, two tubes are slid one inside the other to give almost play-free seating of the outer tube on the inner tube. The double tube thus formed is then placed in a first internal high-pressure forming die and, once the die has been closed, is subjected to internal high pressure in such a way that, at the location of a freely projecting branch in the die cavity, double-walled material of the double tube is forced into this branch to form a double-walled neck. After the relief of the pressure on the pressurized fluid, the internal high-pressure forming die is then opened and the formed double tube is removed. The double tube is then placed in a second internal high-pressure forming die, the cross section of the cavity of which is enlarged close to the ends of the double tube compared with the cross section of the cavity of the first forming die, the enlargement extending over the entire length of the cavity, including the branch. Once the second forming die has been closed and the ends of the double tube have been sealed by axial rams in a clamping action, the double tube is once again subjected to internal high pressure. The inner tube, which has perforations outside the clamping location, remains undeformed by virtue of the pressure balance that is established between the interior of the inner tube and the inside of the outer tube owing to the perforations, while only the outer tube is expanded by the internal high pressure and comes to rest against the cavity of the second internal high-pressure forming die, following its contours accurately. Owing to the indicated difference in the behaviour of the inner tube and the outer tube in relation to the internal high pressure, i.e. the exclusive expansion of the outer tube, a gap is formed on all sides between the clamped ends of the double tube. Once the forming of the double tube in the second forming die has taken place and the double tube has been removed after the opening of the die, an air gap is formed. In the case of exhaust lines as an example, this air gap is intended to insulate the outer tube and the surroundings of the exhaust line, which are accessible to anyone, from the heat of the exhaust gas, which is transferred to the inner tubes that carry the hot gas. This air gap is also intended to ensure an early response from the downstream catalytic converter when cold starting by reducing heat dissipation from the inner tube to the surroundings.
However, the known embodiment described above involves complex apparatus since two dies have to be used to form the double-walled tube. It also requires an undesirably long process time for the overall forming process due to the transfer between the two dies of the workpiece to be formed, the opening time of the first die and the closing time of the second die, and the pressure build-up time in both dies.
An object of the invention is to provide a method of the above-described type such that double-walled hollow sections with an enlarged cross section and with an air gap between the inner and the outer hollow section can be produced in a simple manner in a reduced process time.
This object is achieved according to the invention disclosed and claimed below.
According to one preferred method of the present invention, an inner hollow section is positioned in an outer hollow section, forming a gap which is filled by an intermediate layer and thus forming a double-walled hollow section. After expansion of the double-walled section by internal high-pressure forming, the intermediate layer may be removed from between the inner hollow section and the outer hollow section to create an air gap therebetween.
By means of the intermediate layer between the individual hollow sections that form the double-walled hollow section after being slid one inside the other, the invention creates the prerequisites for an air gap between the hollow sections without the need to carry out internal high-pressure forming for this purpose. In this context, an appropriate choice of dimensions must be made for the cross sections of the two hollow sections to enable an air gap to form in an appropriate manner at a later stage after the internal high-pressure forming that produces the enlargement in the cross section of the two hollow sections and after the intermediate layer is dissolved away. The intermediate layer makes the two hollow sections virtually integral, allowing forming for the purpose of enlarging their cross section to be accomplished uniformly and in a reliable process despite the spacing of the two hollow sections. The intermediate layer need only be dissolved away in a simple manner, the positioning of the hollow sections relative to one another resulting from the forming process either being retained unaltered by end clamping, with no further means being employed, or, where clamping is not used, being maintained by simple holding means at the ends. Thus, the air gap is created by dissolving away the intermediate layer.
Since, according to the invention, the formation of the air gap does not require any forming of the hollow sections, only a single forming die and a single forming step are required to produce the hollow section with air-gap insulation by means of the spacing of the individual hollow sections. The single die and forming step are used solely for the purpose of enlarging the cross section. Thus, due to the elimination of a further forming step, the process time and hence costs for the production of the hollow section are significantly reduced.
Expedient refinements of the invention can be taken from the subclaims; the invention is furthermore explained in greater detail below by means of a number of exemplary embodiments illustrated in the drawings: